Field of the Invention
The present invention relates to a liquid developer for use in image-forming apparatuses that utilize an electrophotographic system, for example, electrophotography, electrostatic recording, and electrostatic printing.
Description of the Related Art
An electrophotographic system is a method in which printed material is obtained by uniformly charging the surface of an image bearing member such as a photosensitive member (charging step), forming an electrostatic latent image by photoexposure on the surface of the image bearing member (photoexposure step), developing the thereby formed electrostatic latent image with a developer that contains colored resin particles (development step), transferring the developer image to a recording medium such as paper or plastic film (transfer step), and fixing the transferred developer image to the recording medium (fixing step).
The developers here are broadly classified into dry developers and liquid developers: colored resin particles formed of a material that contains a binder resin and a colorant such as a pigment are used in a dry state in the former, while the colored resin particles are dispersed in an electrically insulating liquid in the latter.
The need for color output and high-speed printing from image-forming apparatuses that use an electrophotographic system, e.g., copiers, facsimile machines, printers, and so forth, has been increasing in recent years. Within the realm of color printing, the demand for high-resolution, high-quality images has resulted in demand for developers that can accommodate high-speed printing while having the ability to form high-resolution, high-quality images.
Liquid developers are known to be developers that offer advantages with regard to color image reproducibility. With a liquid developer, the occurrence of aggregation by the colored resin particles in the liquid developer during storage is suppressed, and due to this a microfine toner particle can be used. As a consequence, excellent properties with regard to the reproducibility of fine line images and the reproducibility of gradations are readily obtained with a liquid developer. Development is becoming quite active with regard to high-image-quality, high-speed digital printing apparatuses that utilize electrophotographic technologies that, by exploiting these excellent features, carry out charging of the toner particles in a liquid developer and development and transfer of the developer by electrophoresis. In view of these circumstances, there is demand for the development of liquid developers that have even better properties.
Dispersions of colored resin particles in electrically insulating liquids, e.g., hydrocarbon organic solvents, silicone oils, and so forth, are already known as liquid developers. However, the image quality can be substantially reduced when the electrically insulating liquid remains on the recording medium, e.g., paper, plastic film, and so forth, and it has thus been necessary to remove the electrically insulating liquid. In a method generally used to remove the electrically insulating liquid, thermal energy is applied and the electrically insulating liquid is removed by evaporation. However, this method has not been favorable from an environmental perspective or an energy conservation perspective due to the potential for the emission of organic solvent vapors from the apparatus and due to the large energy requirements.
As a countermeasure here, methods have been proposed in which the electrically insulating liquid is cured by photopolymerization. Photocurable liquid developers use a reactive functional group-bearing monomer as the electrically insulating liquid and further contain a dissolved photopolymerization initiator. This photocurable liquid developer can also accommodate high speeds because it undergoes cure through the reaction of the reactive functional group under exposure to light, e.g., ultraviolet radiation. Such a photocurable liquid developer is proposed in Japanese Patent Application Laid-open No. 2003-57883.
Acrylate monomer, e.g., urethane acrylate, is provided as an example of the reactive functional group-bearing monomer in Japanese Patent Application Laid-open No. 2003-57883.
Japanese Patent No. 3442406 proposes the use as the curable electrically insulating liquid of a curable liquid vehicle having a special range of resistance values. Cationically polymerizable-type curable developers, e.g., epoxy compounds, vinyl ethers, cyclic vinyl ethers, and so forth, are given as examples of the curable liquid vehicle.
In addition, Japanese Patent Application Laid-open No. 2015-127812 provides an example of an ultraviolet-curable liquid developer that avoids a decline in volume resistivity and that exhibits balance between the fixing performance and a high image density; this is achieved through the use of a vinyl ether monomer for the cationically polymerizable liquid monomer and through the combination therewith of a specific polymerization initiator.